The present invention relates to an improved process for the production of polyisocyanates with biuret structures.
The production of polyisocyanates with biuret structures by the direct reaction of excess amounts of organic diisocyanates with organic diamines at raised temperatures is known. For example, DE-OS No. 2,261,065 (Example 16) discloses the reaction of excess amounts of 1,6-diisocyanatohexane with 1,6-diaminohexane, during which the reactants are stirred for a period of 12 hours at 180.degree. C. This extensive heating at a high temperature is not only uneconomic but leads to discoloration of the reaction product, particularly under large-scale production conditions. Use of this product is non-fade lacquers is therefore limited. In fact, even after the product of Example 16 of this disclosure was reworked, it was not possible to obtain a monomeric starting diisocyanate-free biuret polyisocyanate completely free of insoluble gel-like side products.
In the method disclosed in DE-OS 2,609,995,gaseous diamine is introduced into the already present diisocyanate at a temperature of from 100.degree. to 250.degree. C. No polyurethane precipitation occurs during this process and the reaction mixture is a clear solution at every point in time. This is achieved by the continuous dilution of the diamines introduced in gaseous form. Although this process allows the production of high-valency polyisocyanates with biuret structures, it is not suitable for carrying out on an industrial scale because of the large volumes of gaseous diamines necessary and the extremely critical control of the reaction conditions.
EP-B-3,505 discloses a process in which the diamine is introduced into the diisocyanate already present with the help of a smooth jet nozzle of defined size under excess pressure. Reaction temperatures of up to 250.degree. C. may be used. Depending upon the reaction temperature, urea dispersions may form in excess starting diisocyanate during the process of this prior publication. Subsequent heat treatment converts such dispersions to solutions of biuret polyisocyanate in excess starting diisocyanate. One disadvantage of this process, in addition to the required use of special apparatus (smooth jet nozzle), is that the resulting polyisocyanates with biuret structures after the removal of the excess starting diisocyanate (in particular 1,6-diisocyanatohexane) have a considerable portion of higher oligomers and side products. This leads to an increase in the viscosity, an undesired decrease of the NCO content, and a worsened dilutability with covalent solvents.